The "NotThem" (as referred to by Adelard Dekker) is a shapechanging creature affiliated with the Entity known as The Stranger. It has the ability to alter memories of those who knew its victims, as well as digital photos. It then replaces them, living an apparently normal life, until moving on to a new victim. For each victim shown, one person appears to be immune to its reality-distorting effects. In several cases it takes a great deal of pleasure in tormenting these people.
In its natural form, it is described as "long and thin", with "stick-like limbs" and "too many joints". Even when encountered in a monstrous form, it was described as looking similar to its victim but "stretched out".
In 1994, Rose Cooper is replaced by the NotThem in the form of Not-Rose. Only her daughter, Lucy Cooper, is aware of the change (MAG 77), and the real Rose Cooper's voice remains unchanged on tape recordings.
In 2001, Lawrence Moore's cousin Carl is replaced by the NotThem. Only Lawrence notices, and only a few Polaroid photos still show the real Carl. Adelard Dekker binds the entity, in the form of Not-Carl, to a web patterned table (MAG 78).
In 2006, Amy Patel sees the NotThem become Not-Graham due to Graham's possession of the table (MAG 3).
In 2016, Breekon & Hope deliver the table to the Magnus Institute (MAG 35). Sasha James is replaced by Not-Sasha after escaping to the Institute's Artefact Storage during Prentiss' attack (MAG 39). In 2017, Jonathan Sims becomes suspicious of Not-Sasha after some unusual comments from Melanie King. He breaks the table trying to destroy the creature, but only succeeds in releasing it. He is pursued into the tunnels beneath the institute by it (MAG 77, MAG 78, MAG 79). The creature is trapped in the tunnels by Jurgen Leitner in MAG 79.
In MAG 165 Not-Sasha encounters John and Martin in the realm of The Stranger, where she insults Sasha. John becomes enraged and he calls upon the Ceaseless Watcher, using the powers of The Eye to destroy her.